This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Formation density is a measurement frequently obtained during well logging. One manner of determining the formation density may be referred to as a gamma density measurement, which involves detecting the extent to which gamma rays scatter through the formation. Conventionally, obtaining a gamma density measurement has involved irradiating the formation with gamma rays using a radioisotopic source (e.g., 137Cs or 241AmBe). These gamma rays may Compton scatter off the electrons present in the formation. Depending on the amount of Compton scattering, some of these gamma rays may be detected by a gamma ray detector spaced some distance from the gamma ray source. Since the concentration of electrons in the formation is proportional to the atomic number of the elements of the formation, and the degree to which the gamma rays Compton scatter and are detected by the gamma ray detector relates to the electron concentration, the density of the formation may be determined based on the count rate of gamma rays detected.
Using radioisotopic sources in a downhole tool may be undesirable, so techniques have been developed to generate photons for a formation density measurement without radioisotopic gamma ray sources. One such technique is referred to as a neutron-gamma density (NGD) measurement, as distinguished from a conventional gamma-gamma density (GGD) measurement. An NGD measurement involves emitting neutrons into the formation using a neutron generator. Some of these neutrons may inelastically scatter off certain elements in the formation, generating inelastic gamma rays that may enable a formation density determination. Although an NGD measurement based on these gamma rays may be accurate in some formations, the NGD measurement may be less accurate in other formations.